The subject of second-language Welsh is being abolished under plans to overhaul Wales’ school curriculum.
All pupils will have to study for first-language Welsh in a dramatic escalation of Welsh-language education in Wales.
The body representing school leaders warned care must be taken not to "disadvantage" children who are not natural Welsh speakers.
The Association of School and College Leaders Cymru also joined the National Education Union saying significant funds would be needed.
Intensive Welsh language training for teachers and teaching assistants is now being planned to ensure they can deliver the changes coming in from 2022.
Radical changes proposed to the way that Welsh is taught mean the language will remain compulsory for all learners aged three to 16 – alongside English - but no longer separated into first and second language programmes of study.
All learners will follow the same curriculum and there will be more of an emphasis on improving their skills and use of the language, the Welsh Government said.
The changes mark the most dramatic shift in the way languages are taught in Welsh schools since the introduction of the National Curriculum 30 years ago, said Education Secretary Kirsty Williams.
But the National Education Union warned "significant development" is needed to realise the ambition with only 34% of registered teachers in Wales identifying as Welsh speakers, which does not necessarily mean they can teach in Welsh.